During drilling or production of oil and gas wells, it is not uncommon for a string of conduit such as casing, drill pipe, coiled tubing, or other conduit, which is downhole, to become lodged within a wellbore at some point along its length. Therefore, there are devices known in the art which can be lowered into the conduit string, and which, through either chemicals or heat imparted to the conduit wall, can cut the conduit such that the portion of the string above the cut can be retrieved from the wellbore and the “stuck” portion below the cut can be abandoned in the wellbore.
One such device is disclosed in U.S. Pat. No. 4,598,769, entitled “Pipe Cutting Apparatus,” by an inventor of the invention described in the present application, Michael Robertson, which patent is incorporated herein by reference. FIG. 1 illustrates a cutting assembly 10 disclosed in the '769 patent. The cutting assembly 10 includes a cutting apparatus 31 (which may be referred to as a “torch”) for producing a cutting fluid. Prior to a cutting operation, the cutting assembly 10 is lowered into a wellbore conduit 12 (i.e., a conduit to be cut) using a lowering device such as wireline 20. To initiate a cutting operation, an electric current is produced from the surface of the well and is applied, through electric conductors, to an electrode plug 30, from where it is conducted on to prong 32, conductor 34, spring 36, and squib 38, all positioned within the cutting portion of the cutting apparatus 31. Loosely packed pyrotechnic material 40 contained within the cutting apparatus 31 is ignited by the current, which, in turn, ignites compressed combustible pyrotechnic material 40 that is formed into pellets 42 surrounding the material 40. A cutting fluid that is produced by the ignited material 40 is directed toward the lower end 43 of the cutting apparatus 31. The fluid is then directed to a nozzle assembly 44, comprising a plurality of cutting nozzles 46, each nozzle 46 configured to direct the conduit cutting fluid 48 from the direction along the elongate axis of cutting assembly 10 radially against the interior wall 25 of conduit 12 to produce a cut along cutline 50.
The reaction of the pyrotechnic material generates a large volume of fluid (e.g., gaseous reaction products) within the conduit at the point of the cut. This fluid volume can create forces that act upon the cutting assembly 10. For example, the discharge of the fluid below the cutting assembly 10 (i.e., downhole) generates a thrust that acts to move the cutting apparatus in an upward direction. Because this action occurs during the cutting operation, if the cutting apparatus is not properly anchored, it may produce very uneven, jagged, or incomplete cuts, and, in extreme cases, may be propelled out of the wellbore, causing damage, and perhaps endangering the safety of the workers on the rig floor.
The cutting apparatus disclosed in the '769 patent employs a mechanical anchoring assembly 16 to address these issues. The anchor assembly 16 includes a series of jaws 22, which extend outwardly (phantom view in FIG. 1) from the body 17 of anchor assembly 16 to engage against the interior wall 25 of the conduit 12. This primary anchoring system is activated when the cutting assembly 10 is positioned downhole at the desired depth prior to the cutting operation to assure the stable positioning of the cutting assembly 10. As a secondary anchoring system, there is provided a series of drag springs 26, which engage against the interior wall 25 of the conduit 12, above the jaws 22, for stabilizing the cutting assembly 10 during the cutting operation. Additional details regarding the operation of the cutting apparatus 31 are described in the '769 patent.
There are certain shortcomings associated with the mechanical anchoring assembly 16. For example, the anchors must be set manually and properly prior to operating the cutting apparatus, which requires preliminary preparation within the wellbore prior to the cutting operation. In addition, due to the mechanical nature of the anchor system, the anchors may not set properly, which may result in the above-described cutting and safety concerns. Still further, the anchors may not function at all, which would require the cutting apparatus to be retrieved from the wellbore for service prior to the cutting operation.
To overcome the shortcomings of the disclosed mechanical anchoring system, Robertson disclosed an improved anchoring system in U.S. Pat. No. 5,435,394, entitled “Anchor system for pipe cutting apparatus,” which patent is incorporated herein by reference. FIG. 2 illustrates an improved anchor assembly 110 that was disclosed in the '394 patent. Anchor assembly 110 acts to stabilize the cutting apparatus 104 during cutting operations as will be described below. The cutting apparatus 104 functions in a substantially similar way as the cutting apparatus 31 of cutting assembly 10, described above, and as more fully described in the '769 patent. The lower end of the cutting apparatus 104 is modified (as compared to cutting apparatus 31) to accommodate the anchor assembly 110. As illustrated, the anchor assembly 110 includes a substantially cylindrical elongated anchor body 112. In the depicted embodiment, the anchor body 112 is threadably attached to the lower end 113 of the cutting apparatus 104 via a threaded pin 116 that is engaged into a threaded port 118 in both the anchor assembly 110 and the cutting apparatus 104. In other embodiments, different mechanisms (e.g., bolts, pins, etc.) are utilized to couple the cutting apparatus 104 to the anchor assembly 110. The lower end 120 of the anchor body 112 is formed into a conical point 122, which eases the process of lowering the cutting apparatus 104 into the wellbore conduit 12.
The external diameter of the anchor body 112 is shown to be substantially equal to the external diameter of the cutting apparatus 104. Therefore, the annular space 124 formed between the anchor body 112 of the anchor assembly 110 and the interior wall 25 of the conduit 12 is shown to be substantially equal to the annular space 126 between the body of the cutting apparatus 104 and the interior wall 25 of the conduit 12. Further, the overall length of the anchor body 112 may vary; however, it is preferred that the overall length be generally equal to the overall length of the cutting apparatus 104, for reasons to be explained below.
Turning now to the anchoring functions of the anchor body 112, when the firing mechanism of the cutting apparatus 104 has been activated, and the ignitor material 40, 42 is producing the cutting fluid (arrow 48) that extends radially outward from the nozzles 46 to cut the wall 14 of the conduit 12, a great volume or “bubble” of the fluid (illustrated by arrows 140) can be produced. This fluid can flow within the annuli 124, 126 formed between the anchor body 112 and the interior wall 25 of the conduit 12, and between the body of the cutting apparatus 104 and the interior wall 25 of the conduit 12, respectively. Because of its length and diameter, the anchor body 112 defines the annular space 124 between itself and the interior wall 25 of the conduit 12 that is substantially equal to the annular space 126 defined between the body of the cutting apparatus 104 and the interior wall 25 of the conduit 12. Therefore, as fluids travel both upward (arrows 144) and downward (arrows 146), the volume of fluid above the cutline 50 is equal to the volume of fluid below the cutline 50, and is of equal pressure. Therefore, applying Boyle's Law, because the two annuli 124, 126 formed by the body of the cutting apparatus 104 and the body 112 of the anchor assembly 110 are substantially equal, the volumes of the fluid in each of the respective annuli 124, 126 are equal because the pressure of the fluid above the cutline 50 is essentially equal to the pressure of the same fluid below the cutline 50. Thus, the resulting downward forces due to thrust and the pressure of the fluids above the cutline 50 are equal to the upward forces due to thrust and the pressure of the fluids below the cutline 50. The resulting forces on the torch are therefore equalized, which acts to maintain the position of the cutting apparatus 104 relative to the conduit 12.
When the annuli 124, 126 are substantially equal, the lengths of the anchor body 112 and the cutting apparatus 104 should also be substantially equal in order to equalize the volumes in the annuli 124, 126, but this is not always required. What is required is that the resulting volume contained within the upper annulus 126 be equal to the volume contained within the lower annulus 124 such that the upward and downward forces are equalized.
Because the cutting procedure takes place within a time frame of less than a second, the equalization of pressures must be present only during that time to ensure that the cutline 50 is as smooth and straight as possible. Should the fluid volumes eventually change, causing a pressure imbalance after the completion of the cutting operation, the position of the cutting apparatus 104 may shift. However, as long as the cut has been completed, the shifting of the cutting apparatus 104 becomes immaterial. Moreover, the initial high pressures of the fluid, during and immediately following the cut, will have been reduced rather rapidly, thus avoiding any possibility of causing the torch to travel upward at a dangerous speed within the hole.
While the improved anchoring apparatus 110 balances the upward and downward forces on the cutting apparatus 104 such that the axial position of the cutting apparatus 104 within the conduit 12 is maintained for the duration of a cutting operation, there are additional problems associated with the cutting efficiency associated with prior art cutting devices. One common problem associated with prior art conduit cutting devices occurs as a result of the arrangement of multiple discrete nozzles (e.g., nozzles 46) about the cutting apparatus. During cutting operations, as the fluid is expelled from the nozzles, the fluid tends to cut or perforate the conduit wall at a target location directly across the annular gap between the nozzle and the conduit. This cutting action often leaves uncut conduit material between the cut holes or perforations at the target locations, resulting in the conduit remaining intact. This results in the need to deploy another cutting device into the conduit to complete the cutting operation.
Another problem associated with prior art conduit cutting devices is caused by the pivoting or swinging action of the cutting apparatus during cutting operations. Specifically, as the fluid is expelled from the nozzles, slight differences in forces within the annuli 124, 126 caused by the moving fluid may move or pivot the cutting device off its central longitudinal axis. For example, the cutting device may swing side to side, from a wireline, within the conduit that is being cut. Such movements cause the discharged cutting fluid to contact the inner conduit wall above or below the intended target location, resulting in an uneven cut. If the movement of the cutting device is significant, the heat energy of the fluid may be applied to and dissipated over a large surface area of the conduit wall, which may result in an incomplete cut.
Given these shortcomings, the art of oilfield tools, and, specifically, wellbore conduit cutting devices, would benefit from improved methods and apparatus for maintaining the position of a cutting device within a wellbore while also producing safer and more consistent cuts.